Diagnostic medical imaging systems typically include a scan portion and a control portion having a display. For example, ultrasound imaging systems usually include ultrasound scanning devices, such as ultrasound probes having transducers that are connected to an ultrasound system to control the acquisition of ultrasound data by performing various ultrasound scans (e.g., imaging a volume or body). The ultrasound probes typically include an array or matrix of transmit/receive elements, which transmit ultrasound waves and receive back-scattered echo signals. The ultrasound systems are controllable to operate in different modes of operation and to perform different scans. The received signals are then processed to generate images for display to a user.
Different beamforming techniques may be used to synthetically modify the effective transmit beam used by ultrasound systems to acquire ultrasound data that is used to generate the images. For example, Retrospective Transmit Beamforming (RTB) is used to form a synthetically focused ultrasound image using standard, scanned, and focused ultrasound transmissions. More particularly, RTB is a synthetic focus technique that uses standard, scanned-beam transmit data, dynamic receive focusing, and combination of time-aligned data from multiple transmits to form images.
One variation of the standard architecture of an ultrasound system employs a dynamically focused, multi-line acquisition (MLA) beamformer, which produces multiple receive beamformed output signals for each transmit. Such a system allows a broad transmit beam to be used to illuminate the reflectivity distribution, while more than one narrow receive beam is used to produce data for image generation. Two-stage RTB is an implementation of the RTB technique wherein the first stage implements high-MLA receive beamforming and the second stage implements a combination of receive data from different transmits. In known systems, the receive data is recorded in straight lines, with successive samples representing increasing depths at the same beam steering angle. Also in known systems, the data is aligned using some form of model of the high-power portion of the physical transmit beam, which limits the application of RTB at the transmit focal depth.
Moreover, in these known systems, the number of transmit events (or transmits) to combine at every image location is dictated by the applicability of the data alignment scheme. In general, RTB solves the problem of de-focusing of an ultrasound image at depths different from the transmit focal depth. In some other known systems, this issue is addressed by performing multiple transmits in each look direction, each focused at a different depth, and using each to form a separate depth zone of the image. However, this slows the frame rate, and, thus, negatively affects the overall performance of the system.